Compounds with Chirality Centers Other than Carbon

The necessary criterion that an object not be superimposable on its mirror image can be met by compounds in which the chiral center is other than tetracoordinate carbon. Many such examples are known, including sulfoxides in which the substituents on sulfur are different. These molecules are nonplanar, with significant barriers to pyramidal inversion. 

An effect observed with a number of compounds which have apparent chiral centers on elements other than carbon. Eor example, secondary and tertiary amines have a pyramidal structure in which the unshared pair of electrons is at the top of the pyramid. If the three substituents hnked to the nitrogen are all different, one might suspect that the tertiary amine would give rise to optical activity and be resolvable. However, rapid oscillation of the unshared pair of electrons on one side of the nitrogen to the other (hence, pyramidal inversion) in effect causes interconversion of the two enantiomers and prevents resolution. If the nitrogen is at a bridgehead, this umbrella effect is inhibited and optical isomers can be isolated. 

Since the most common cause of chirality is the presence of four different substituents bonded to a tetrahedral atom, tetrahedral atoms other than carbon can also be chirality centers. Silicon, nitrogen, phosphorus, and suit fur are all commonly encountered in organic molecules, and all can be chin rality centers under the proper circumstances. We know, for example, thal trivalent nitrogen is tetrahedral, with its lone pair of electrons acting a4 the fourth substituent (Section 1.11). Is trivalent nitrogen chiral Does compound such as ethylmethylamine exist as a pair of enantiomers j... 

Compounds with Chirality Centers Other than Carbon 232... 

COMPOUNDS WITH CHIRALITY CENTERS OTHER THAN CARBON... 

It is important to note that the 2" rule predicts only the maximum number of stereoisomers possible in compounds with more than one center of chirality. For example, some compounds with two asymmetrically substituted carbon atoms may have only three stereoisomeric forms. This occurs when three of the substituents on one asymmetric carbon are the same as those on the other asymmetric carbon, as shown... 

Atoms other than asymmetric carbons can be chirality centers. When an atom such as nitrogen or phosphorus has four different groups or atoms attached to it and it has a tetrahedral geometry, it is a chirality center. A compound with a chirality center can exist as enantiomers, and the enantiomers can be separated. 

---